Air feed compression is one of the more costly components of air separation. Air compression is required in all practical methods of separating air into its constituents to yield useful product streams containing oxygen, nitrogen, or argon. Examples of air separation processes requiring pressurized air feed include pressure swing adsorption, chemical absorption, cryogenic distillation, permeation by polymeric membranes, and diffusion through solid ceramic ion transport membranes. Each of these processes are carried out in a system generically known as an air separation unit or ASU.
Minimizing capital costs and maximizing operating efficiency in air compression usually require the use of constant speed, motor-driven axial or centrifugal air compressors. An important operating characteristic of these compressors is that the mass flow of air which can be compressed varies significantly when parameters such as suction air temperature, suction air pressure, air humidity, and intercooler heat sink temperature change over diurnal or seasonal time periods. As air density decreases due to an environmental change such as increased ambient air temperature, the mass flow discharged from the compressor decreases. The decrease may be caused by a flow limit imposed by the essentially constant inlet volume capacity of the compressor as required for peak thermodynamic efficiency, or by a power limit of the motor-driver resulting from the increased power required for compression as air or intercooler heat sink temperatures increase. Control equipment such as inlet guide vanes can be used to increase motor-compressor operating flow ranges. Variable speed motor systems or transmissions can be employed to increase the speed of the compressor to increase volumetric capacity as ambient temperature rises; however, the loss of power transmission efficiency and the high cost of variable speed equipment usually precludes the use of these systems.
An air separation process yields at least two product streams--one enriched in a given air constituent and another depleted in that constituent. In the commercial operation of air separation processes, the stream enriched in a desired constituent is a primary product which is utilized in separate consuming process. The stream depleted in the desired constituent typically is considered a secondary product or byproduct which has less value or no value relative to the process which utilizes the primary product. For example, an air separation process can be designed and operated to produce a primary product containing 99 mole % oxygen and a byproduct containing up to 6 mole % oxygen. Conversely, a process designed and operated to yield a primary product containing 99.5 mole % nitrogen can yield a byproduct containing greater than 50 mole % oxygen. Since air separation processes are operated at pressures above atmospheric utilizing a compressed air feed, the byproduct stream often is available at a pressure above atmospheric.
The cost-effective recovery of a primary air separation product in an air separation process can be improved by utilization of the byproduct stream or streams. A byproduct stream can be used for its compositional properties, for example as an oxidant when it is rich in oxygen or a relatively inert diluent when it is rich in nitrogen. Additionally, independent of its composition, the byproduct stream can be work expanded to drive other process machinery and/or to produce refrigeration.
The largest total tonnage of atmospheric gases is produced by the cryogenic separation of air using well-known process cycles. In many cryogenic air separation systems, oxygen is recovered as the primary product and a nitrogen-rich stream is withdrawn as a byproduct. In the separation of air for integrated gasification combined cycle (IGCC) power generation systems, oxygen is the main product and is used for fuel gas production, while nitrogen is a byproduct which is introduced into the gas turbine combustor for energy recovery and combustion moderation. This application is described in the review papers entitled "Improved IGCC Power Output and Economics Incorporating a Supplementary Gas Turbine" by A. R. Smith et al presented at the 13.sup.th EPRI Conference on Gasification Power Plants, Oct. 19-21, 1994, San Francisco, and "Integrated Gasification Combined Cycle Performance Testing" by R. L. Bannister et al in PWR-Vol. 32, Proceedings of the Joint Power Generation Conference, Book No. G01073, 1997.
The production of oxygen by air separation for synthesis gas generation in alternate fuels production also yields a byproduct nitrogen stream which can be used in a gas turbine combustor for combustion moderation, expanded to generate work and/or refrigeration, or heated and used for steam production. These applications are described in a paper entitled "Air Separation Unit Integration for Alternative Fuels Projects" by A. R. Smith et al presented at the International Gas Turbine and Aeroengine Congress and Exposition, Stockholm, Jun. 2-5, 1998. Other representative applications for the use of byproduct nitrogen in gas turbines are described in U.S. Pat. Nos. 5,081,845; 5,421,166; 5,251,450; 5,251,541; 5,257,504; and 5,666,823.
Byproduct nitrogen from cryogenic air separation plants also can be used for applications other than combustion moderation. U.S. Pat. No. 5,635,541 discloses the expansion of byproduct nitrogen in an expansion turbine to generate electricity or to produce chilled water for process use. U.S. Pat. No. 5,388,395 also describes the use of byproduct nitrogen in an expansion turbine to generate electricity wherein the reduced-pressure cooled nitrogen is used to cool the gas turbine compressor inlet air. In U.S. Pat. No. 5,406,786, byproduct nitrogen is compressed and used for gas turbine blade cooling and for moderating combustion in the gas turbine combustor. Great Britain Patent Specification 1 455 960 discloses the work expansion of a heated nitrogen byproduct stream to provide some or all of the work to drive a two-stage ASU feed air compressor. Work expansion of a byproduct nitrogen stream to drive one stage of a multiple-stage ASU feed air compressor is described in Czech Patent No. 261114, U.S. Pat. No. 5,560,223, and French Patent Publication 2 686 405.
U.S. Pat. No. 5,040,370 discloses an air separation-chemical process plant integration in which heat from an oxygen-based reaction in the process plant is used to heat the nitrogen byproduct from the ASU, and the heated nitrogen is expanded to produce useful external work. A nitrogen byproduct is work-expanded to produce refrigeration for use within the air separation plant, and the expanded stream is further used for external ambient cooling as described in U.S. Pat. No. 5,146,756. The expansion of byproduct nitrogen optionally drives a nitrogen product compressor.
U.S. Pat. No. 5,740,673 describes an integrated gasification combined cycle power generation system in which air is supplied to the air separation plant in part from the gas turbine air compressor and in part by an independent air compressor. During periods of off-design operation when the gas turbine air compressor cannot supply sufficient compressed air to the air separation plant, supplemental air is provided by the independent air compressor.
A method to supply compressed air to a process plant is described in U.S. Pat. Nos. 5,402,631 and 5,485,719 in which a gas turbine, air compressors, and optionally gas expanders are integrated in an integral-gear gas processing system.
Air can be separated at high temperature using ceramic ion transport membranes in which oxygen is extracted from hot compressed air by permeation through the membrane to produce high-purity oxygen and a hot oxygen-depleted or nitrogen-enriched byproduct stream. This byproduct stream is expanded in a gas turbine after optional additional heating to provide work for air compression and optionally to generate electric power as described in U.S. Pat. Nos. 4,545,787; 5,035,727; 5,516,359; 5,657,624; 5,562,754; and 5,565,017. Alternatively, the byproduct stream can be cooled and used to generate work at lower temperatures as described in U.S. Pat. No. 5,753,007.
U.S. Pat. No. 5,256,172 describes a pressure swing adsorption system in which pressurized intermediate products or byproducts are work-expanded to provide a portion of the work required for feed gas compression.
Because air feed compression is one of the largest components of the total cost of air separation, there is a strong incentive to reduce both the capital and operating costs associated with this process step. The potential for capital cost reduction is especially significant for large air separation plants. The design of an air compression system for an air separation plant is complex because the design must address changes in ambient air temperature, pressure, and humidity which reduce the capacity or efficiency of air compression equipment. In addition, the design often must account for fluctuating product demand which may occur independently of, or simultaneously with, changes in ambient air conditions. An air separation process usually operates at varying ambient air conditions. The process generates one or more byproduct streams which are not of primary commercial value, and it is desirable to utilize these streams in some manner to improve the operation and reduce the cost of air separation, particularly the cost of compression.
The present invention addresses these needs by providing means to increase the overall specific power efficiency of feed air compression, supply the necessary volume of product under variable operating conditions, and utilize byproduct streams in an effective manner to achieve these objectives. The invention is described in detail below and is defined by the claims which follow.